1. Field of the Invention
The present invention relates to a heat-resistant cast steel which can be appropriately applied to component parts or the like for exhaust systems for automobile engines. More particularly, it relates to a ferritic heat-resistant cast steel which has excellent machinability, which simultaneously has good toughness and thermal fatigue resistance and which can be produced at less expensive production cost, and a process for producing the same.
2. Description of the Related Art
Recently, vehicle engines, especially automobile engines, have been improved in terms of combustion in order to fulfill the low fuel consumption requirement and the environment friendly requirement. As a result, the temperature of the exhaust gases tends to increase.
Hence, instead of the high-silicon-content nodular graphite cast iron employed conventionally and widely, ferritic heat-resistant cast steel has come to make the component parts, for the exhaust systems, such as the exhaust manifolds, the turbine housings of the turbo chargers, the component parts of the exhaust gas purifying apparatuses and the like. Although the ferritic heat-resistant cast steel is superior to the high-silicon-content nodular graphite cast iron in terms of heat resistance, it is remarkably inferior thereto in terms of machinability. Accordingly, it pushes up manufacturing cost and deteriorates productivity. In order to improve the machinability of steel, it has been known that the addition of sulfur is effective. For example, as set forth in page 416 of "Stainless Steel Binran (Stainless Steel Handbook in Japan)," an SUS430F steel containing sulfur in an amount of 0.15% by weight or more is available as one of ferritic stainless steels according to JIS (Japanese Industrial Standard).
However, when sulfur is added to the conventional ferritic heat-resistant cast steels, the addition degrades the thermal fatigue resistance of them. Hence, the content of sulfur has been limited to as small as the trace amount of inevitable impurities or less. For example, in SCH1, one of the ferritic heat-resistant cast steels as per JIS, and the ferritic heat-resistant cast steels disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 1-159,355 and Japanese Unexamined Patent Publication (KOKAI) No. 2-175,841, the sulfur content is limited to 0.04% by weight or less. Exceptionally, in Japanese Unexamined Patent Publication (KOKAI) No. 5-59,498, there is disclosed a ferritic heat-resistant cast steel whose matrix includes C in an amount of from 0.05 to 0.5% by weight, Si in an amount of from 1 to 2% by weight and Cr in an amount of from 10 to 20% by weight, and in which a heat-resistance-giving element, such as Nb, V and Mn, is added in an amount of from about 0.1 to 1% by weight. According to this publication, the sulfur content can be increased up to 0.2% by weight.
When sulfur is added to the ferritic heat-resistant cast steel disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 5-59,498 above, the resulting cast steel is improved in terms of machinability as compared to that of SCH1, however, it is inferior thereto in terms of toughness and thermal fatigue resistance. For example, it is liable to break when it is subjected to mechanical shocks in the manufacturing processes of the cast products for the exhaust systems. Further, the thus obtained cast products are likely to crack during their service where the tensile thermal stresses act concentratedly.
In addition, the conventional ferritic heat-resistant cast steels free from the sulfur addition were far inferior to the high-silicon-content nodular graphite cast iron in terms of machinability.
Therefore, in order to solve the problems associated with the conventional ferritic heat-resistant cast steels with sulfur added, the inventors of the present invention investigated the variation of the toughness with respect to the sulfur addition, the relationship between the toughness and the sulfides distribution, and the relationship between the resistance against the cracking resulting from the thermal fatigue (hereinafter simply referred to as "thermal fatigue resistance") and the toughness as well as the tensile strength properties.
First, the present inventors examined the deterioration of the toughness due to the sulfur addition by carrying out the Charpy impact test in a temperature range of from room temperature to 300.degree. C. As a result, they appreciated that, in the temperature range of from room temperature to 300.degree. C., the ferritic heat-resistant cast steels with sulfur added have a ductile-brittle transition temperature (hereinafter simply referred to as "transition temperature"), which is generally appreciated in ferritic alloys as set forth in page 154 of "Stainless Steel Binran (Stainless Steel Handbook in Japan)." At the same time, they recognized that the cast steels with sulfur added exhibit a remarkably decreased impact value at the transition temperature or more (hereinafter simply referred to as "intermediate temperature toughness"). Hence, they revealed that it is important to closely observe the intermediate toughness of the cast steels and to inhibit it from deteriorating in order to improve machinability by adding sulfur.
Second, the present inventors studied the relationship between the sulfides distribution and the intermediate temperature toughness in the ferritic heat resistance cast steels with sulfur added. As a result, in the ferritic heat-resistant cast steel disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 5-59,498, the Nb carbides crystallize in interdendritic regions like a network during the solidification with or without the sulfur addition. However, they found that, when sulfur is added, the sulfides crystallize together with the Nb carbides and the interdendritic regions are embrittled, and that the cast steel exhibits a sharply deteriorated intermediate temperature toughness accordingly. Therefore, they realized that, in order to inhibit the deterioration of the intermediate temperature toughness due to the sulfur addition, it is important to uniformly distribute the sulfides crystallizing during the solidification without localizing them. Consequently, they discovered that it is necessary to suppress the Nb addition amount to a small amount which enables not to crystallize the Nb carbides in the interdendritic regions like a network during the solidification.
Third, the present inventors investigated the relationship between the thermal fatigue resistance and the toughness as well as the tensile strength properties. As a result, they noticed that, when sulfur is added to the ferritic heat-resistant cast steels, the cast steels exhibit a decreasing thermal fatigue resistance, but that the cast steels had the virtually invariable tensile strength properties, such as proof stress, tensile strength and elongation, in a temperature range of from room temperature to elevated temperatures, and that there is no correlation between the thermal fatigue resistance and the tensile strength properties. On the other hand, they found that there is a good correspondence between the thermal fatigue resistance deterioration and the intermediate temperature toughness deterioration. Thus, in the ferritic heat-resistant cast steels with sulfur added, they revealed that, in order to upgrade the thermal fatigue resistance, it is extremely important to inhibit the intermediate temperature toughness (hereinafter simply abbreviated to as "toughness") from deteriorating.
Based on the novel discoveries described above, the present inventors aimed at the following. Namely, they preliminarily inquired into the formation processes of the solidified metallic structure of the ferritic heat-resistant cast steel, and the relationship between the crystallizing sulfides distribution and the toughness.
First of all, the present inventors studied the formation processes of the solidified metallic structure of the ferritic heat-resistant cast steel, and they found that, depending on the alloy compositions, there are mainly following four formation processes:
(a) Similarly to the aforementioned Nb carbides, the carbides crystallize in the interdendritic regions during the solidification (hereinafter simply referred to as "carbides crystallizing solidification"); PA1 (b) Only the ferrite phase (hereinafter simply referred to as "alpha") crystallizes during the solidification, and the solidification terminates at the "alpha" single-phase (hereinafter simply referred to as "alpha" single-phase solidification"); PA1 (c) The "alpha" crystallized first as primary crystal, and thereafter part of primary "alpha" and part of the remaining liquid phase cause a peritectic reaction to crystallize the austenite phase (hereinafter simply referred to as "gamma"), and the solidification terminates at the mixed phase of "alpha" and "gamma" (hereinafter simply referred to as "peritectic solidification"); and PA1 (d) Similarly to (c), there occurs the peritectic reaction during the solidification, but the solidification terminates at the "gamma" single-phase (hereinafter simply referred to as ""gamma" single-phase solidification"). PA1 C in an amount of from 0.1 to 0.4% by weight; PA1 Si in an amount of from 0.5 to 2.0% by weight; PA1 Mn in an amount of 1.0% by weight or less; PA1 S in an amount of from 0.06 to 0.20% by weight; PA1 Ni in an amount of 1.0% by weight or less; PA1 Cr in an amount of from 13 to 20% by weight; PA1 V in an amount of from 0.2 to 1.0% by weight; PA1 at least one element selected from the group consisting of Nb in an amount of from 0.1 to 0.4% by weight, Mo in an amount of from 0.1 to 2.0% by weight, and W in an amount of from 0.2 to 4.0% by weight; and PA1 the balance of Fe and inevitable impurities, PA1 the S being dispersed as sulfides in the ferritic heat-resistant cast steel. PA1 selecting and casting a raw material having the composition of the present cast steel or the preferred form thereof; and PA1 annealing the resulting cast product at a temperature of from 750.degree. to 1,000.degree. C.
Then, the present inventors added sulfur to the four ferritic heat-resistant cast steels which had undergone the aforementioned four different solidification processes, and they examined the resulting four cast steels for the relationship between the crystallizing sulfides distribution and the toughness. As a result, they discovered the following:
The cast steel undergone process (a), i.e., the carbides crystallizing solidification, exhibits a sharply deteriorating toughness when the sulfur addition amount is increased, because the sulfides crystallize along the carbides and densely localize in the interdendritic regions in a manner similar to the aforementioned crystallization of the Nb carbides;
The cast steel undergone process (b), i.e., the "alpha" single-phase solidification, exhibits a sharply deteriorating toughness when the sulfur addition amount is increased, because the sulfides mainly crystallize along the "alpha" crystalline grain boundaries and densely localize therein so as to embrittle them;
The cast steel undergone process (d), i.e., the "gamma" single-phase solidification, exhibits a sharply deteriorating toughness when the sulfur addition amount is increased, because the sulfides mainly crystallize along the "gamma" crystalline grain boundaries and densely localize therein so as to embrittle them; and
On the other hand, the cast steel undergone process (c), i.e., the peritectic solidification, exhibits a toughness which is inhibited from deteriorating even when the sulfur addition amount is increased, because the sulfides do not localize along the specific structures but distribute uniformly therein.
Based on these novel discoveries, the present inventors designed a ferritic heat-resistant cast steel so as to cause the peritectic solidification, and so as to heighten the eutectoid transformation temperature and elevated temperature proof stress which effect the thermal fatigue resistance, and then they added sulfur to the resulting cast steel. Thus, they completed a ferritic heat-resistant cast steel which is superior to the conventional steels free from sulfur addition in terms of machinability, and which, in spite of the sulfur addition capable of improving the machinability equivalent to that of the conventional steels with sulfur added, is tougher and more thermal fatigue resistant than the conventional steels.